Optimal Bracing System for Steel Towers

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A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications

(IJERA) ISSN: 2248-9622 www.ijera.com

Vol. 3, Issue 2, March -April 2013, pp.729-732

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Optimal Bracing System for Steel Towers

A. Jesumi*, M.G. Rajendran***(PG student, School of Civil Engineering, Karunya University, Coimbatore641 114)

** (Professor, School of Civil Engineering, Karunya University, Coimbatore641 114)

ABSTRACTThe major system providing lateral load

resistance in steel lattice towers is bracing system.

There are different types of bracing systems for

steel lattice towers. The heights of these towersvary from 20 to 500 meters, based on the practical

requirements. This study has focused on

identifying the economical bracing system for a

given range of tower heights. Towers of height 40m

and 50m have been analyzed with different typesof bracing systems under wind loads. The diagonal

wind has been found to be the maximum for

towers. The optimal bracing system has been

identified and reported.Keywords - bracing system, steel lattice towers,wind analysis

1. INTRODUCTIONTowers are tall steel framework construction

used for different purposes such as communication,radio transmission, satellite receptions, air trafficcontrols, television transmission, power transmission,

flood light stands, oil drilling masts, meteorologicalmeasurements, etc. The present paper discussesmicrowave transmission towers. Lattice towers act as

vertical trusses and resists wind load by cantileveraction. The bracing members are arranged in manyforms, which carry solely tension, or alternativelytension and compression. The bracing is made up of

crossed diagonals, when it is designed to resist onlytension. Based on the direction of wind, one diagonaltakes all the tension while the other diagonal is

assumed to remain inactive. Tensile bracing is smallerin cross-section and is usually made up of a back-to-back channel or angle sections. The bracings behave

as struts, when it is designed to take compression.One of the most common arrangements is the crossbracing. The most significant dimension of a tower isits height. It is normally several times larger than the

horizontal dimensions. The area which is occupied atthe ground level is considerably limited and so,slender structures are commonly used.The tapered part of the tower is advantageous with

regard to the bracing, as it reduces the design forces.The greater the height of the tower, greater will be thedistance it can transmit radio signals. Towers areclassified as Self-supporting towers and Guyedtowers. Self-supporting towers are generally preferred

since they require less base area. Towers are subjectedto gravity loads and horizontal loads.

Bracings hold the structure stable by transferring theloads sideways (not gravity, but wind or earthquake

loads) down to the ground and are used to resistlateral loads, thereby preventing sway of the structure.Bracing increases the resistance of the structureagainst side sway or drift. The higher the structure,

the more it is exposed to lateral loads such as windload, since it has higher tendency to sway. If thebracing is weak, the compression member would

buckle which leads to failure of the tower. Diagonalbraces are efficient elements for developing stiffnessand resistance to wind loads. There are different types

of bracing systems in common use such as Singlediagonal bracing, double diagonal (X-X) bracing, X-Bbracing, XBX bracing, arch bracing, subdivided Vbracing, diamond lattice system of bracing, K, Y, W,

X bracings, etc.K. Agarwal and K. Garg (1994) have assessed free-standing lattice towers for wind loads. It has beenfound that large variations have occurred in windloads on towers and there are several gaps in the

present recommendations which are to be answeredby more rigorous wind tunnel investigations. The

behavior of cross-bracings in latticed towers wasstudied by Alan R. Kemp and Roberto H. Behncke(1998). The cross bracings have shown complex

behavior and the number of bolts in the connection ofthe bracing to the main legs have been apparent in theresults. M.Selvaraj, S.M.Kulkarni and R.RameshBabu (2012) have investigated on the behavior of

built up transmission line tower from FiberReinforced Polymer (FRP) pultruded sections. They

have discussed experimental studies carried out on anX-braced panel of transmission line tower made from

FRP pultruded sections. The upgradation oftransmission towers using a diaphragm bracing

system was experimented by F. Albermani, M.Mahendran

and S. Kitipornchai (2004). Their resultsshowed that considerable strength improvements wereachieved with diaphragm bracings. The upgrading

system using the most efficient diaphragm bracingtype has been successfully implemented on anexisting 105 m-height TV tower. F. Al-Mashary, A.Arafah and G. H. Siddiqi [5] have investigated on theeffective bracing of trussed towers against secondarymoments. The study showed that improper bracing

configuration of the main and/or secondary braces

induced high secondary moments. Thus, it isnecessary to identify the economical bracing systemfor a given range of tower height.


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The main purpose of the paper is to demarcate theeconomical bracing system of steel lattice towers.

Investigations carried out to analyze towers withdifferent heights and different bracing configurationshave been presented. The towers have been analyzed

for wind loads with STAAD Pro., to compare themaximum joint displacement of each tower.Optimized design has been carried out to estimate and

to compare the weight of each tower. The results havebeen used to identify the economical bracing systemfor a given range of height of towers.

2. GENERAL CONSIDERATIONSIn this study, five steel lattice towers with

different bracing configurations such as the X-B,single diagonal, X-X, K and Y bracings have beenmodeled for a given range of height. The heights ofthe towers are 40m and 50m with a base width of 2m

and 5m respectively. The tower of height 40m has 13panels and the tower of height 50m has 16 panels. 70-72% of the height is provided for the tapered part and

28-30% of the height is provided for the straight partof the tower.

3. TOWER ANALYSISThe towers have been modeled using

geometric coordinates. The member property was

assigned to each member of the structure. The leg,diagonal bracings and horizontal bracings areprovided with angle sections. Elastic modulus,Poissons ratio, density, alpha and damping are the

material properties used for the analysis. Ahemispherical dome was assumed to be mounted atthe top panels of the towers. The towers were

analyzed considering it to act as a space structure withpin joints, for dynamic wind loading and optimizeddesign was carried out. The displacements andweights of the towers obtained were compared toarrive at an optimal solution.

Fig. 1: Models of the towers with different bracingconfigurations for 40m height

The elevations of the towers of height 40m and 50mwith different bracing configurations are shown in

fig.1 and 2 respectively.

Fig. 2: Models of towers with different bracing

configurations for 50m height

4. LOADINGThe loads act in three mutually perpendicular

directions such as vertical, normal to the face andparallel to the face of the tower. The loads applied tothe towers are based on the codal provisions in IS:

875-1987 part 3. Since towers are tall and flexible, itis critical under wind load. The wind load isdetermined by dividing the tower into different panelsof equal heights. The calculated wind load is

transferred on each joint of the exposed face of thetower. For square steel lattice towers, the maximum

load occurs when wind blows diagonally. Therefore,IS: 875-1987 part 3 recommends the diagonal wind tobe 1.2 times the wind blowing normal to the face. Thevertical loads act centrally and are distributed equallyamong the four legs. Bending moments produce an

equal compression in the two legs of one side, andequal tension in the two legs of the other side whenthe wind is considered acting in any one direction.

The shear forces are resisted by the horizontalcomponent of the leg forces and the brace forces.Thus, the taper has a major influence on the design ofthe bracing.

The design wind pressure is calculated at increasing

heights of a tower from the mean ground level withthe following equation:

PZ= 0.6VZ2 (1)

Where,PZis the design wind pressure in N/m

2

VZis the design wind speed in m/s

The design wind speed is obtained taking intoaccount, factors such as risk coefficient, terrain

roughness, height, size of the structure and localtopography. It is expressed as:

VZ= Vbk1k2k3 (2)

Where,Vbis the basic wind speed in m/s


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k1is the risk coefficientk2 is the terrain, height and structure size

factork3is the local topography factor

The basic wind speed for the proposed location was

39m/s. The wind load is the product of the dynamicwind pressure, the overall force coefficient and theeffective exposed area of the tower. The force

coefficient for the exposed surface depends on thesolidity ratio. It is expressed as:

F = CF.A.PZ (3)Where,

F is the force acting on the structureCFis the force coefficientA is the exposed surface area of the structurePZis the design wind pressure

5. LOAD COMBINATION

The towers have been analyzed for the followingload combinations according to IS: 875-1987 part 3.

1.

DL + WL with wind blowing normal to the

face of the tower2. DL + WL with wind blowing diagonal to the

face of the tower3.

6. ANALYSISThe steel lattice towers have been analyzed,

idealizing it as a 3D truss as per IS: 800-2007, with

various bracing configurations. The dead loads actingon the tower are self weight of the tower and selfweight of antenna. The wind loads are considered

acting both normal and diagonal to the face of thetower. Wind loads have been computed by MS Exceland have been incorporated in the analysis done by

STAAD Pro.V8i. The joint displacement with respectto normal wind and diagonal wind has been obtainedfrom the analysis for each tower and has beentabulated and shown in table 1. The sizes of leg and

bracing members have been checked for themaximum forces computed from the analysis andoptimized designed has been done as per IS: 800-2007

and the weight of each tower has been noted andshown in table 2.

7. RESULTS AND DISCUSSIONTable 1: Maximum joint displacement of the tower

Table 2: Results showing the weight of the tower

0

50

100

150

200

40 50Weightofthetower

Height of the tower

X-B

Single Diagonal

X-X

K

Y

Fig. 3: Weight of each tower

From the above results in table 2 and fig. 3, it hasbeen found that Y bracing is the most appropriate

arrangement of bracing system that resists lateralloads for the given range of heights of the towers, as itshows comparatively lesser weight than the otherbracing systems. The demarcated economical bracing

system is shown in fig. 4.

Fig. 4: The economical bracing system of tower

The displacement undergone by the tower is shown infig. 5.


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Fig. 5: Displacement of the tower

8. CONCLUSIONIn this paper, analytical studies have been

presented to find the most appropriate arrangementand cost-effective bracing system of steel lattice

towers for the effective resistance against lateralforces. The joint displacement and weights are thesignificant parameters obtained from the analysis.

However, there is no sufficient data regarding thepermissible displacement for towers. From the resultsobtained, Y bracing has been found to be the mosteconomical bracing system up to a height of 50m.Further, the study will be carried out for towers ofgreater heights.

ACKNOWLEDGEMENTThe authors thank the management of KarunyaUniversity for having provided the necessary facilitiesto carry out this work.

REFERENCE[1]

K. Agarwal and K. Garg, Wind LoadAssessment on Free Standing Lattice Towers,

Indian Institute of Engineers Journal, vol 75,November 1994,pp. 171-177.

[2]

Alan R. Kemp and Roberto H. Behncke,Behavior of Cross-Bracing in Latticed Towers,Journal of Structural Engineering, April 1998,pp. 360-367.

[3]

M.Selvaraj, S.M.Kulkarni and R.Ramesh Babu,

Behavioral Analysis Of Built Up TransmissionLine Tower From FRP Pultruded Sections,International Journal of Emerging Technology

and Advanced Engineering, Volume 2, Issue 9,September 2012,ISSN 2250-2459.

[4]

F. Albermani, M. Mahendran

and S.Kitipornchai, Upgrading of TransmissionTowers Using a Diaphragm Bracing System,

Engineering Structures, 26,pp. 735-744.

[5]

F. Al-Mashary, A. Arafah and G. H. Siddiqi,Effective Bracing of Trussed Towers against

Secondary Moments,faculty.ksu.edu.sa/2639/Publications%20PDF/TOWER-BR.DOC.

[6]

Indian Standard Code of Practice for designloads (other than earthquake) for buildings andstructures IS: 875-1987, part 3 Reaffirmed 1997

Second Revision), 1989. Bureau of IndianStandards, New Delhi.

[7]

Indian Standard Code of Practice for designloads (other than earthquake) for buildings and

structures IS: 875-1987, part 5 Reaffirmed 2008Second Revision), 1988. Bureau of IndianStandards, New Delhi.

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Optimal Bracing System for Steel Towers